CMOS image sensor including stacked semiconductor chips and readout circuitry including a superlattice

ABSTRACT

A CMOS image sensor may include a first semiconductor chip including an array of image sensor pixels and readout circuitry electrically connected thereto, and a second semiconductor chip coupled to the first semiconductor chip in a stack and including image processing circuitry electrically connected to the readout circuitry. The readout circuitry may include a plurality of transistors each including spaced apart source and drain regions, a superlattice channel extending between the source and drain regions, and a gate including a gate insulating layer on the superlattice channel and a gate electrode on the gate insulating layer.

TECHNICAL FIELD

The present disclosure generally relates to semiconductor devices and,more particularly, to CMOS structures and related circuits and methods.

BACKGROUND

Structures and techniques have been proposed to enhance the performanceof semiconductor devices, such as by enhancing the mobility of thecharge carriers. For example, U.S. Patent Application No. 2003/0057416to Currie et al. discloses strained material layers of silicon,silicon-germanium, and relaxed silicon and also including impurity-freezones that would otherwise cause performance degradation. The resultingbiaxial strain in the upper silicon layer alters the carrier mobilitiesenabling higher speed and/or lower power devices. Published U.S. PatentApplication No. 2003/0034529 to Fitzgerald et al. discloses a CMOSinverter also based upon similar strained silicon technology.

U.S. Pat. No. 6,472,685 B2 to Takagi discloses a semiconductor deviceincluding a silicon and carbon layer sandwiched between silicon layersso that the conduction band and valence band of the second silicon layerreceive a tensile strain. Electrons having a smaller effective mass, andwhich have been induced by an electric field applied to the gateelectrode, are confined in the second silicon layer, thus, an n-channelMOSFET is asserted to have a higher mobility.

U.S. Pat. No. 4,937,204 to Ishibashi et al. discloses a superlattice inwhich a plurality of layers, less than eight monolayers, and containinga fractional or binary or a binary compound semiconductor layer, arealternately and epitaxially grown. The direction of main current flow isperpendicular to the layers of the superlattice.

U.S. Pat. No. 5,357,119 to Wang et al. discloses a Si—Ge short periodsuperlattice with higher mobility achieved by reducing alloy scatteringin the superlattice. Along these lines, U.S. Pat. No. 5,683,934 toCandelaria discloses an enhanced mobility MOSFET including a channellayer comprising an alloy of silicon and a second materialsubstitutionally present in the silicon lattice at a percentage thatplaces the channel layer under tensile stress.

U.S. Pat. No. 5,216,262 to Tsu discloses a quantum well structurecomprising two barrier regions and a thin epitaxially grownsemiconductor layer sandwiched between the barriers. Each barrier regionconsists of alternate layers of SiO2/Si with a thickness generally in arange of two to six monolayers. A much thicker section of silicon issandwiched between the barriers.

An article entitled “Phenomena in silicon nanostructure devices” also toTsu and published online Sep. 6, 2000 by Applied Physics and MaterialsScience & Processing, pp. 391-402 discloses a semiconductor-atomicsuperlattice (SAS) of silicon and oxygen. The Si/O superlattice isdisclosed as useful in a silicon quantum and light-emitting devices. Inparticular, a green electroluminescence diode structure was constructedand tested. Current flow in the diode structure is vertical, that is,perpendicular to the layers of the SAS. The disclosed SAS may includesemiconductor layers separated by adsorbed species such as oxygen atoms,and CO molecules. The silicon growth beyond the adsorbed monolayer ofoxygen is described as epitaxial with a fairly low defect density. OneSAS structure included a 1.1 nm thick silicon portion that is abouteight atomic layers of silicon, and another structure had twice thisthickness of silicon. An article to Luo et al. entitled “Chemical Designof Direct-Gap Light-Emitting Silicon” published in Physical ReviewLetters, Vol. 89, No. 7 (Aug. 12, 2002) further discusses the lightemitting SAS structures of Tsu.

Published International Application WO 02/103,767 A1 to Wang, Tsu andLofgren, discloses a barrier building block of thin silicon and oxygen,carbon, nitrogen, phosphorous, antimony, arsenic or hydrogen to therebyreduce current flowing vertically through the lattice more than fourorders of magnitude. The insulating layer/barrier layer allows for lowdefect epitaxial silicon to be deposited next to the insulating layer.

Published Great Britain Patent Application 2,347,520 to Mears et al.discloses that principles of Aperiodic Photonic Band-Gap (APBG)structures may be adapted for electronic bandgap engineering. Inparticular, the application discloses that material parameters, forexample, the location of band minima, effective mass, etc., can betailored to yield new aperiodic materials with desirable band-structurecharacteristics. Other parameters, such as electrical conductivity,thermal conductivity and dielectric permittivity or magneticpermeability are disclosed as also possible to be designed into thematerial.

Furthermore, U.S. Pat. No. 6,376,337 to Wang et al. discloses a methodfor producing an insulating or barrier layer for semiconductor deviceswhich includes depositing a layer of silicon and at least one additionalelement on the silicon substrate whereby the deposited layer issubstantially free of defects such that epitaxial silicon substantiallyfree of defects can be deposited on the deposited layer. Alternatively,a monolayer of one or more elements, preferably comprising oxygen, isabsorbed on a silicon substrate. A plurality of insulating layerssandwiched between epitaxial silicon forms a barrier composite.

Despite the existence of such approaches, further enhancements may bedesirable for using advanced semiconductor materials and processingtechniques to achieve improved performance in semiconductor devices.

SUMMARY

A CMOS image sensor may include a first semiconductor chip including anarray of image sensor pixels and readout circuitry electricallyconnected thereto, and a second semiconductor chip coupled to the firstsemiconductor chip in a stack and including image processing circuitryelectrically connected to the readout circuitry. The readout circuitrymay include a plurality of transistors each including spaced apartsource and drain regions and a superlattice channel extending betweenthe source and drain regions. The superlattice channel may include aplurality of stacked groups of layers, with each group of layersincluding a plurality of stacked base semiconductor monolayers defininga base semiconductor portion, and at least one non-semiconductormonolayer constrained within a crystal lattice of adjacent basesemiconductor portions. Each transistor may further include a gateincluding a gate insulating layer on the superlattice channel and a gateelectrode on the gate insulating layer.

More particularly, the first semiconductor chip may further include anelectrical interconnect layer beneath the array of image sensor pixelsand defining a back side illumination (BSI) configuration therewith,with the electrical interconnect layer electrically connecting the arrayof image sensor pixels with the readout circuitry. Moreover, theelectrical interconnect layer may include a semiconductor layer and aplurality of spaced apart conductive traces within the semiconductorlayer.

The CMOS image sensor may further include at least one lens overlyingthe array of image sensor pixels, as well as at least one color filteroverlying the array of image sensor pixels. By way of example, the atleast one color filter may comprise a respective color filter for eachof the pixels in the array of image sensor pixels. Also by way ofexample, the at least one color filter may comprise a plurality ofdifferent color filters for filtering different respective wavelengthsof light.

In an example implementation, the CMOS image sensor may further includea third semiconductor chip coupled with the first and secondsemiconductor chips in the stack and including a plurality of memorycircuits. Furthermore, the image processing circuitry may also include aplurality of transistors each including a superlattice channel. By wayof example, the at least one non-semiconductor monolayer may compriseoxygen, and the semiconductor monolayers may comprise silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged schematic cross-sectional view of asuperlattice for use in a semiconductor device in accordance with anexample embodiment.

FIG. 2 is a perspective schematic atomic diagram of a portion of thesuperlattice shown in FIG. 1.

FIG. 3 is a greatly enlarged schematic cross-sectional view of anotherembodiment of a superlattice in accordance with an example embodiment.

FIG. 4A is a graph of the calculated band structure from the gamma point(G) for both bulk silicon as in the prior art, and for the 4/1 Si/Osuperlattice as shown in FIGS. 1-2.

FIG. 4B is a graph of the calculated band structure from the Z point forboth bulk silicon as in the prior art, and for the 4/1 Si/O superlatticeas shown in FIGS. 1-2.

FIG. 4C is a graph of the calculated band structure from both the gammaand Z points for both bulk silicon as in the prior art, and for the5/1/3/1 Si/O superlattice as shown in FIG. 3.

FIG. 5 is an exploded perspective view of a CMOS image sensor device inaccordance with an example embodiment including stacked semiconductorchips and readout circuitry including a superlattice.

FIG. 6 is an exploded perspective view of another CMOS image sensordevice in accordance with an example embodiment including stackedsemiconductor chips and image processing including a superlattice.

FIG. 7 is an exploded perspective view of still another CMOS imagesensor device in accordance with an example embodiment including stackedsemiconductor chips with a memory circuit chip and readout/imageprocessing circuitry including a superlattice.

FIG. 8 is a cross-sectional diagram of a transistor including asuperlattice channel which may be used in the circuits of the devices ofFIGS. 5-7 in an example embodiment.

FIG. 9 is a cross-sectional diagram of an example CMOS image sensorpixel configuration which may be used in the devices of FIG. 5-7.

FIGS. 10 and 11 are graphs of simulated yield I_(W) vs. yield SNM,respectively, for a prior art transistor configuration and a transistorwith a superlattice channel providing a super steep retrograde profilein accordance with an example embodiment.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings, in which the example embodimentsare shown. The embodiments may, however, be implemented in manydifferent forms and should not be construed as limited to the specificexamples set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete. Like numbers referto like elements throughout, and prime and multiple prime notation areused to indicate similar elements in different embodiments.

Generally speaking, the present disclosure relates to CMOS image sensor(CIS) devices having an enhanced semiconductor superlattice thereinwhich may provide desired speed enhancement and thermal managementfeatures. The enhanced semiconductor superlattice is also referred to asan “MST” layer or “MST technology” in this disclosure and theaccompanying drawings.

More particularly, the MST technology relates to advanced semiconductormaterials such as the superlattice 25 described further below. Applicanttheorizes, without wishing to be bound thereto, that certainsuperlattices as described herein reduce the effective mass of chargecarriers and that this thereby leads to higher charge carrier mobility.Effective mass is described with various definitions in the literature.As a measure of the improvement in effective mass Applicant's use a“conductivity reciprocal effective mass tensor”, M_(e) ⁻¹ and M_(h) ⁻¹for electrons and holes respectively, defined as:

${M_{e,i,j}^{- 1}\left( {E_{F},T} \right)} = \frac{\sum\limits_{E > E_{F}}{\int_{B.Z.}^{\;}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\ \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}\ d^{3}k}}}{\sum\limits_{E > E_{F}}{\int_{B.Z.}^{\;}{{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}d^{3}k}}}$for electrons and:

${M_{h,i,j}^{- 1}\left( {E_{F},T} \right)} = \frac{- {\sum\limits_{E < E_{F}}{\int_{B.Z.}^{\;}{\left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{i}\ \left( {\nabla_{k}{E\left( {k,n} \right)}} \right)_{j}\frac{\partial{f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}}{\partial E}\ d^{3}k}}}}{\sum\limits_{E < E_{F}}{\int_{B.Z.}^{\;}{\left( {1 - {f\left( {{E\left( {k,n} \right)},E_{F},T} \right)}} \right)d^{3}k}}}$for holes, where f is the Fermi-Dirac distribution, E_(F) is the Fermienergy, T is the temperature, E(k,n) is the energy of an electron in thestate corresponding to wave vector k and the n^(th) energy band, theindices i and j refer to Cartesian coordinates x, y and z, the integralsare taken over the Brillouin zone (B.Z.), and the summations are takenover bands with energies above and below the Fermi energy for electronsand holes respectively.

Applicant's definition of the conductivity reciprocal effective masstensor is such that a tensorial component of the conductivity of thematerial is greater for greater values of the corresponding component ofthe conductivity reciprocal effective mass tensor. Again Applicanttheorizes without wishing to be bound thereto that the superlatticesdescribed herein set the values of the conductivity reciprocal effectivemass tensor so as to enhance the conductive properties of the material,such as typically for a preferred direction of charge carrier transport.The inverse of the appropriate tensor element is referred to as theconductivity effective mass. In other words, to characterizesemiconductor material structures, the conductivity effective mass forelectrons/holes as described above and calculated in the direction ofintended carrier transport is used to distinguish improved materials.

Applicant has identified improved materials or structures for use insemiconductor devices. More specifically, Applicant has identifiedmaterials or structures having energy band structures for which theappropriate conductivity effective masses for electrons and/or holes aresubstantially less than the corresponding values for silicon. Inaddition to the enhanced mobility characteristics of these structures,they may also be formed or used in such a manner that they providepiezoelectric, pyroelectric, and/or ferroelectric properties that areadvantageous for use in a variety of different types of devices, as willbe discussed further below.

Referring now to FIGS. 1 and 2, the materials or structures are in theform of a superlattice 25 whose structure is controlled at the atomic ormolecular level and may be formed using known techniques of atomic ormolecular layer deposition. The superlattice 25 includes a plurality oflayer groups 45 a-45 n arranged in stacked relation, as perhaps bestunderstood with specific reference to the schematic cross-sectional viewof FIG. 1.

Each group of layers 45 a-45 n of the superlattice 25 illustrativelyincludes a plurality of stacked base semiconductor monolayers 46defining a respective base semiconductor portion 46 a-46 n and an energyband-modifying layer 50 thereon. The energy band-modifying layers 50 areindicated by stippling in FIG. 1 for clarity of illustration.

The energy band-modifying layer 50 illustratively includes onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions. By “constrained within a crystallattice of adjacent base semiconductor portions” it is meant that atleast some semiconductor atoms from opposing base semiconductor portions46 a-46 n are chemically bound together through the non-semiconductormonolayer 50 therebetween, as seen in FIG. 2. Generally speaking, thisconfiguration is made possible by controlling the amount ofnon-semiconductor material that is deposited on semiconductor portions46 a-46 n through atomic layer deposition techniques so that not all(i.e., less than full or 100% coverage) of the available semiconductorbonding sites are populated with bonds to non-semiconductor atoms, aswill be discussed further below. Thus, as further monolayers 46 ofsemiconductor material are deposited on or over a non-semiconductormonolayer 50, the newly deposited semiconductor atoms will populate theremaining vacant bonding sites of the semiconductor atoms below thenon-semiconductor monolayer.

In other embodiments, more than one such non-semiconductor monolayer maybe possible. It should be noted that reference herein to anon-semiconductor or semiconductor monolayer means that the materialused for the monolayer would be a non-semiconductor or semiconductor ifformed in bulk. That is, a single monolayer of a material, such assilicon, may not necessarily exhibit the same properties that it wouldif formed in bulk or in a relatively thick layer, as will be appreciatedby those skilled in the art.

Applicant theorizes without wishing to be bound thereto that energyband-modifying layers 50 and adjacent base semiconductor portions 46a-46 n cause the superlattice 25 to have a lower appropriateconductivity effective mass for the charge carriers in the parallellayer direction than would otherwise be present. Considered another way,this parallel direction is orthogonal to the stacking direction. Theband modifying layers 50 may also cause the superlattice 25 to have acommon energy band structure, while also advantageously functioning asan insulator between layers or regions vertically above and below thesuperlattice.

Moreover, this superlattice structure may also advantageously act as abarrier to dopant and/or material diffusion between layers verticallyabove and below the superlattice 25. These properties may thusadvantageously allow the superlattice 25 to provide an interface forhigh-K dielectrics which not only reduces diffusion of the high-Kmaterial into the channel region, but which may also advantageouslyreduce unwanted scattering effects and improve device mobility, as willbe appreciated by those skilled in the art.

It is also theorized that semiconductor devices including thesuperlattice 25 may enjoy a higher charge carrier mobility based uponthe lower conductivity effective mass than would otherwise be present.In some embodiments, and as a result of the band engineering achieved bythe present invention, the superlattice 25 may further have asubstantially direct energy bandgap that may be particularlyadvantageous for opto-electronic devices, for example.

The superlattice 25 also illustratively includes a cap layer 52 on anupper layer group 45 n. The cap layer 52 may comprise a plurality ofbase semiconductor monolayers 46. The cap layer 52 may have between 2 to100 monolayers of the base semiconductor, and, more preferably between10 to 50 monolayers.

Each base semiconductor portion 46 a-46 n may comprise a basesemiconductor selected from the group consisting of Group IVsemiconductors, Group III-V semiconductors, and Group II-VIsemiconductors. Of course, the term Group IV semiconductors alsoincludes Group IV-IV semiconductors, as will be appreciated by thoseskilled in the art. More particularly, the base semiconductor maycomprise at least one of silicon and germanium, for example.

Each energy band-modifying layer 50 may comprise a non-semiconductorselected from the group consisting of oxygen, nitrogen, fluorine, carbonand carbon-oxygen, for example. The non-semiconductor is also desirablythermally stable through deposition of a next layer to therebyfacilitate manufacturing. In other embodiments, the non-semiconductormay be another inorganic or organic element or compound that iscompatible with the given semiconductor processing as will beappreciated by those skilled in the art. More particularly, the basesemiconductor may comprise at least one of silicon and germanium, forexample

It should be noted that the term monolayer is meant to include a singleatomic layer and also a single molecular layer. It is also noted thatthe energy band-modifying layer 50 provided by a single monolayer isalso meant to include a monolayer wherein not all of the possible sitesare occupied (i.e., there is less than full or 100% coverage). Forexample, with particular reference to the atomic diagram of FIG. 2, a4/1 repeating structure is illustrated for silicon as the basesemiconductor material, and oxygen as the energy band-modifyingmaterial. Only half of the possible sites for oxygen are occupied in theillustrated example.

In other embodiments and/or with different materials this one-halfoccupation would not necessarily be the case as will be appreciated bythose skilled in the art. Indeed it can be seen even in this schematicdiagram, that individual atoms of oxygen in a given monolayer are notprecisely aligned along a flat plane as will also be appreciated bythose of skill in the art of atomic deposition. By way of example, apreferred occupation range is from about one-eighth to one-half of thepossible oxygen sites being full, although other numbers may be used incertain embodiments.

Silicon and oxygen are currently widely used in conventionalsemiconductor processing, and, hence, manufacturers will be readily ableto use these materials as described herein. Atomic or monolayerdeposition is also now widely used. Accordingly, semiconductor devicesincorporating the superlattice 25 in accordance with the invention maybe readily adopted and implemented, as will be appreciated by thoseskilled in the art.

It is theorized without Applicant wishing to be bound thereto that for asuperlattice, such as the Si/O superlattice, for example, that thenumber of silicon monolayers should desirably be seven or less so thatthe energy band of the superlattice is common or relatively uniformthroughout to achieve the desired advantages. The 4/1 repeatingstructure shown in FIGS. 1 and 2, for Si/O has been modeled to indicatean enhanced mobility for electrons and holes in the X direction. Forexample, the calculated conductivity effective mass for electrons(isotropic for bulk silicon) is 0.26 and for the 4/1 SiO superlattice inthe X direction it is 0.12 resulting in a ratio of 0.46. Similarly, thecalculation for holes yields values of 0.36 for bulk silicon and 0.16for the 4/1 Si/O superlattice resulting in a ratio of 0.44.

While such a directionally preferential feature may be desired incertain semiconductor devices, other devices may benefit from a moreuniform increase in mobility in any direction parallel to the groups oflayers. It may also be beneficial to have an increased mobility for bothelectrons and holes, or just one of these types of charge carriers aswill be appreciated by those skilled in the art.

The lower conductivity effective mass for the 4/1 Si/O embodiment of thesuperlattice 25 may be less than two-thirds the conductivity effectivemass than would otherwise occur, and this applies for both electrons andholes. Of course, the superlattice 25 may further comprise at least onetype of conductivity dopant therein, as will also be appreciated bythose skilled in the art.

Indeed, referring now additionally to FIG. 3, another embodiment of asuperlattice 25′ in accordance with the invention having differentproperties is now described. In this embodiment, a repeating pattern of3/1/5/1 is illustrated. More particularly, the lowest base semiconductorportion 46 a′ has three monolayers, and the second lowest basesemiconductor portion 46 b′ has five monolayers. This pattern repeatsthroughout the superlattice 25′. The energy band-modifying layers 50′may each include a single monolayer. For such a superlattice 25′including Si/O, the enhancement of charge carrier mobility isindependent of orientation in the plane of the layers. Those otherelements of FIG. 3 not specifically mentioned are similar to thosediscussed above with reference to FIG. 1 and need no further discussionherein.

In some device embodiments, all of the base semiconductor portions of asuperlattice may be a same number of monolayers thick. In otherembodiments, at least some of the base semiconductor portions may be adifferent number of monolayers thick. In still other embodiments, all ofthe base semiconductor portions may be a different number of monolayersthick.

In FIGS. 4A-4C, band structures calculated using Density FunctionalTheory (DFT) are presented. It is well known in the art that DFTunderestimates the absolute value of the bandgap. Hence all bands abovethe gap may be shifted by an appropriate “scissors correction.” Howeverthe shape of the band is known to be much more reliable. The verticalenergy axes should be interpreted in this light.

FIG. 4A shows the calculated band structure from the gamma point (G) forboth bulk silicon (represented by continuous lines) and for the 4/1 Si/Osuperlattice 25 shown in FIG. 1 (represented by dotted lines). Thedirections refer to the unit cell of the 4/1 Si/O structure and not tothe conventional unit cell of Si, although the (001) direction in thefigure does correspond to the (001) direction of the conventional unitcell of Si, and, hence, shows the expected location of the Si conductionband minimum. The (100) and (010) directions in the figure correspond tothe (110) and (−110) directions of the conventional Si unit cell. Thoseskilled in the art will appreciate that the bands of Si on the figureare folded to represent them on the appropriate reciprocal latticedirections for the 4/1 Si/O structure.

It can be seen that the conduction band minimum for the 4/1 Si/Ostructure is located at the gamma point in contrast to bulk silicon(Si), whereas the valence band minimum occurs at the edge of theBrillouin zone in the (001) direction which we refer to as the Z point.One may also note the greater curvature of the conduction band minimumfor the 4/1 Si/O structure compared to the curvature of the conductionband minimum for Si owing to the band splitting due to the perturbationintroduced by the additional oxygen layer.

FIG. 4B shows the calculated band structure from the Z point for bothbulk silicon (continuous lines) and for the 4/1 Si/O superlattice 25(dotted lines). This figure illustrates the enhanced curvature of thevalence band in the (100) direction.

FIG. 4C shows the calculated band structure from both the gamma and Zpoint for both bulk silicon (continuous lines) and for the 5/1/3/1 Si/Ostructure of the superlattice 25′ of FIG. 3 (dotted lines). Due to thesymmetry of the 5/1/3/1 Si/O structure, the calculated band structuresin the (100) and (010) directions are equivalent. Thus the conductivityeffective mass and mobility are expected to be isotropic in the planeparallel to the layers, i.e. perpendicular to the (001) stackingdirection. Note that in the 5/1/3/1 Si/O example the conduction bandminimum and the valence band maximum are both at or close to the Zpoint.

Although increased curvature is an indication of reduced effective mass,the appropriate comparison and discrimination may be made via theconductivity reciprocal effective mass tensor calculation. This leadsApplicant to further theorize that the 5/1/3/1 superlattice 25′ shouldbe substantially direct bandgap. As will be understood by those skilledin the art, the appropriate matrix element for optical transition isanother indicator of the distinction between direct and indirect bandgapbehavior.

Referring now to FIG. 5, the above described superlattice structures mayadvantageously be used in a CMOS image sensor 100. The image sensor 100illustratively includes a first semiconductor chip 101 which in turnincludes an array of image sensor pixels 102 and readout circuitry 103electrically connected thereto. The sensor 100 further illustrativelyincludes a second semiconductor chip 104 coupled to the firstsemiconductor chip 101 in a stack and including image processingcircuitry 105 electrically connected to the readout circuitry.

An example pixel cell architecture which may be used for the imagesensor pixel array 102 is shown in FIG. 9. In the illustrated example,the array 102 is a back side illumination (BSI) configuration whichillustratively includes a plurality of photodiodes 106 with insulatingregions 107 therebetween. Furthermore, an electrical interconnect layer108 is beneath the array of image sensor pixels 102 in the illustratedBSI configuration (although the stacked chip CIS arrangements describedherein may be used with front side illumination (FSI) configurations aswell). The electrical interconnect layer 108 illustratively includes asemiconductor layer 109 (e.g., polysilicon) and a plurality of spacedapart conductive traces 110 electrically connecting the array of imagesensor pixels 102 with the readout circuitry 103. By way of example, thearray 102 may include CMOS active pixel sensors with pinned photodiodes106 and associated control/output circuitry (e.g., a 4T cell).

Furthermore, a respective red, green, or blue color filter 111R, 111G,or 111B may be positioned above each of the photodiodes 106, as well asa respective micro-lens 112 overlying each of the color filters. Thelenses 112 may be used to advantageously gather and direct light to eachof the photodiodes 106. The signal detected by each photodiode 106 maybe individually read out by a row and column selector, followed byamplification circuitry (not shown) and the readout circuitry 103 toprovide the signals to the image processing circuitry 105.

Generally speaking, BSI implementations may be desirable over front sideillumination (FSI) configurations for a variety of reasons. First, BSIsensors typically have higher light sensitivity and higher quantumefficiency (QE) or approximately 70-80%. Moreover, in BSI sensors theelectrical interconnect layer 108 is out of the optical path, whichallows for a thinner substrate and results in less optical crosstalk.Furthermore, BSI configurations allow for a wider chief ray angle (CRA),which enables large aperture lenses and thinner modules.

Nevertheless, in a typical BSI CIS integration the pixel transistors andimage processing circuits are fabricated on separate wafers (i.e., thefirst and second semiconductor chips 101, 104) using different processtechnologies. Generally speaking, more advanced technologies are usedfor fabricating the image processing circuitry 105 on the second chip104. For example, two different technology nodes may be used, such as 40nm for the second semiconductor wafer 105 and 65 nm or 90 nm for thefirst semiconductor chip 101. While this may provide for processing andcost savings with respect to the first semiconductor chip 101, use ofthe different nodes may cause transistor performance mismatches to occurbetween the readout circuitry 103 and the more advanced, high-speedimage processing circuitry 105. This, in turn, may degrade productperformance.

In accordance with an example implementation, the readout circuitry 103may advantageously include a plurality of transistors (MOSFETs) 20 (FIG.8) including the above-described band-engineered superlattice 25. Moreparticularly, the MOSFET 20 illustratively includes a substrate 21,source/drain regions 22, 23, source/drain extensions 26, 27, and achannel region therebetween provided by the superlattice 25.Source/drain silicide layers 30, 31 and source/drain contacts 32, 33overlie the source/drain regions, as will be appreciated by thoseskilled in the art. Regions indicated by dashed lines 34, 35 areoptional vestigial portions formed originally with the superlatticematerial, but thereafter heavily doped. In other embodiments, thesevestigial superlattice regions 34, 35 may not be present as will also beappreciated by those skilled in the art.

A gate 38 illustratively includes a gate insulating layer 37 adjacentthe channel provided by the superlattice 25, and a gate electrode layer36 on the gate insulating layer. Sidewall spacers 40, 41 are alsoprovided in the illustrated MOSFET 20. It should be noted that othertransistor configurations including the above-described superlatticematerial may also be used in different embodiments for the readoutcircuitry 103 in addition to the planar MOSFET 20 shown. Moreover, someportion of the channel may also be defined in the substrate 21 incertain implementations. Further details on the MOSFET 20 may be foundin U.S. Pat. No. 6,897,472 to Mears et al., which is assigned to thepresent Applicant and hereby incorporate herein in its entirety byreference.

In particular, as described above the band-engineered MST materialadvantageously helps enhance charge carrier flow and thereby increasecircuit speed, and thus total product performance, which helps alleviatethe above-described mismatch between the readout circuitry 103 and theimage processing circuitry 105. Moreover, use of the MST superlatticematerial also provides for a significant Vt variability improvement inthe readout circuitry 103 transistors, as will be discussed furtherbelow, which may also advantageously help reduce fixed pattern noise inthe image sensor 100 as well. By way of example, the superlatticechannels 25 in the transistors 20 of the readout circuitry may be formedusing selective epitaxy at the appropriate locations, or by blanketdeposition followed by patterning of the blanket superlattice layer.

By way of reference, as an example of the speed enhancements which maybe achieved using transistors with MST channels in logic circuits,simulations of various logic circuit configurations with and withoutMST-enabled transistors were performed. The first logic circuit was aninverter, and the simulations showed that the MST-enabled invertor hadan approximate 20% decrease in leakage with a 15% increase in speed andcomparable power consumption. Further, NAND logic gates with and withoutMST-enabled transistors were also simulated, and the MST-enabled gatehad approximately 10% lower leakage with an approximate 20% increase inspeed and a 5% lower power consumption. For the simulations, a 4/1repeating MST material was used for the channel layer (as seen in FIG.1).

In the typical BSI process flow, photodiode 106 implantation may firstbe performed in a silicon substrate or layer, followed by the poly/metaldeposition for the electrical interconnect layer 108. Thereafter, thewafer may be flipped over and bonded to a silicon handle. Backsidegrinding may then be performed to reveal the photodiodes 106, and thecolor filters 111R, 111G, 1113 and micro-lenses 112 may be providedthereafter to complete the image sensor pixel array 102.

Referring now additionally FIG. 6, another example embodiment of theimage sensor 100′ illustratively includes image processing circuitry105′ which has one or more sections which also include transistors (suchas the transistor 20 of FIG. 8) having a superlattice channel. That is,in this example the MST material is used in the image processingcircuitry 105′ of the second semiconductor chip 104′ rather than in thereadout circuitry 103′ of the first semiconductor chip 101′ (although itmay be used in both in some embodiments, as will be discussed furtherbelow). By way of example, the image processing circuitry 105′ mayinclude a plurality of counters, and the transistors 20 may be used todefine the counters, although they may be used in other areas of theimage processing circuitry as well (e.g., logic/processing circuitry,etc.).

By way of background, temporal noise due to thermal noise is animportant characteristic for CIS devices. One approach for reducingthermal noise is to reduce heat dissipation using low-power circuitswith low-voltage operation. More particularly, use of the MST materialto provide super steep retrograde (SSR) channel formation as a result ofits inherent dopant diffusion blocking effect may advantageously beleveraged to improve Vt variability and allow lower voltage operation.Further details regarding the use of the MST material to provide desiredSSR profiles are provided in U.S. Pat. Pub. Nos. 2016/0336406 and2016/0336407 to Mears et al., which are also assigned to the presentApplicant and are hereby incorporated herein in their entireties byreference.

Turning now to FIGS. 10 and 11, a comparison is provided between 6T-SRAMyield for uniform doping (graph 120) and an SSR channel resulting froman MST film (graph 121) as simulated by a TCAD modelling. Moreparticularly, the change in a functional window 122 of the graph 120 tothe functional window 123 of the graph 121 demonstrates that the SSRachieved by the MST film may advantageously reduce Vdd from 1V to 0.5V,and thereby reduce heat for lower thermal noise.

Referring additionally to FIG. 7, another example implementation of theCMOS image sensor 100″ illustratively includes a third semiconductorchip 130″ coupled with the first and second semiconductor chips 101″,104″ in the stack (i.e., in between them). Here, the third chip 130″illustratively includes a plurality of memory circuits, which in thepresent example includes DRAM circuitry 131″. Furthermore, in thisexample, both the readout circuitry 103″ and the image processingcircuitry 105″ include transistors with the MST superlattice film toprovide the above-noted operational advantages.

A method for making the CMOS image sensor 100 may include forming thefirst semiconductor chip 101 including the array of image sensor pixels102 and readout circuitry 103 electrically connected thereto, andforming the second semiconductor chip 104 including image processingcircuitry 105 electrically connected to the readout circuitry. Themethod may further include coupling the first semiconductor chip 101 andthe second semiconductor chip 104 together in a stack as shown in FIG.5. The readout circuitry 103 may include a plurality of transistors(such as the MOSFET 20) including a superlattice channel 25 as describedfurther above.

A related method for making the CMOS image sensor 100′ may includeforming the first semiconductor chip 101′ including the array of imagesensor pixels 102′ and readout circuitry 103′ electrically connectedthereto, and forming the second semiconductor chip 104′ including imageprocessing circuitry 105′ electrically connected to the readoutcircuitry. The method may further include coupling the firstsemiconductor chip 101′ and the second semiconductor chip 104′ in astack as shown in FIG. 6. The processing circuitry 105′ may include aplurality of transistors (such as the MOSFET 20) each including asuperlattice channel 25 as discussed further above.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

That which is claimed is:
 1. A CMOS image sensor comprising: a firstsemiconductor chip comprising an array of image sensor pixels andreadout circuitry electrically connected thereto; and a secondsemiconductor chip coupled to the first semiconductor chip in a stackand comprising image processing circuitry electrically connected to thereadout circuitry; the readout circuitry comprising a plurality oftransistors each comprising spaced apart source and drain regions, asuperlattice channel extending between the source and drain regions, thesuperlattice channel comprising a plurality of stacked groups of layers,each group of layers comprising a plurality of stacked basesemiconductor monolayers defining a base semiconductor portion, and atleast one non-semiconductor monolayer constrained within a crystallattice of adjacent base semiconductor portions, and a gate comprising agate insulating layer on the superlattice channel and a gate electrodeon the gate insulating layer; wherein the superlattice channel islocated entirely within the readout circuitry, and wherein nosuperlattice is located within the image processing circuitry.
 2. TheCMOS image sensor of claim 1 wherein the first semiconductor chipfurther comprises an electrical interconnect layer beneath the array ofimage sensor pixels and defining a back side illumination (BSI)configuration therewith, the electrical interconnect layer electricallyconnecting the array of image sensor pixels with the readout circuitry.3. The CMOS image sensor of claim 2 wherein the electrical interconnectlayer comprises a semiconductor layer and a plurality of spaced apartconductive traces within the semiconductor layer.
 4. The CMOS imagesensor of claim 1 further comprising at least one lens overlying thearray of image sensor pixels.
 5. The CMOS image sensor of claim 1further comprising at least one color filter overlying the array ofimage sensor pixels.
 6. The CMOS image sensor of claim 5 wherein the atleast one color filter comprises a respective color filter for each ofthe pixels in the array of image sensor pixels.
 7. The CMOS image sensorof claim 5 wherein the at least one color filter comprises a pluralityof different color filters for filtering different respectivewavelengths of light.
 8. The CMOS image sensor of claim 1 furthercomprising a third semiconductor chip coupled with the first and secondsemiconductor chips in the stack and comprising a plurality of memorycircuits.
 9. The CMOS image sensor of claim 1 wherein the imageprocessing circuitry also comprises a plurality of transistors eachincluding a superlattice channel.
 10. The CMOS image sensor of claim 1wherein the at least one non-semiconductor monolayer comprises oxygen.11. The CMOS image sensor of claim 1 wherein the semiconductormonolayers comprise silicon.
 12. A CMOS image sensor comprising: a firstsemiconductor chip comprising an array of image sensor pixels, anelectrical interconnect layer beneath the array of image sensor pixelsand defining a back side illumination (BSI) configuration therewith, andreadout circuitry electrically connected to the array of image sensorpixels by the electrical interconnect layer; a second semiconductor chipcoupled to the first semiconductor chip in a stack and comprising imageprocessing circuitry electrically connected to the readout circuitry;and a third semiconductor chip coupled with the first and secondsemiconductor chips in the stack and comprising a plurality of memorycircuits; the readout circuitry comprising a plurality of transistorseach comprising spaced apart source and drain regions, a superlatticechannel extending between the source and drain regions, the superlatticechannel comprising a plurality of stacked groups of layers, each groupof layers comprising a plurality of stacked base semiconductormonolayers defining a base semiconductor portion, and at least onenon-semiconductor monolayer constrained within a crystal lattice ofadjacent base semiconductor portions, and a gate comprising a gateinsulating layer on the superlattice channel and a gate electrode on thegate insulating layer; wherein the superlattice channel is locatedentirely within the readout circuitry, and wherein no superlattice islocated within the image processing circuitry.
 13. The CMOS image sensorof claim 12 further comprising at least one lens overlying the array ofimage sensor pixels.
 14. The CMOS image sensor of claim 12 furthercomprising at least one color filter overlying the array of image sensorpixels.
 15. The CMOS image sensor of claim 12 wherein the imageprocessing circuitry also comprises a plurality of transistors eachincluding a superlattice channel.
 16. The CMOS image sensor of claim 12wherein the at least one non-semiconductor monolayer comprises oxygen.17. The CMOS image sensor of claim 12 wherein the semiconductormonolayers comprise silicon.
 18. A CMOS image sensor comprising: a firstsemiconductor chip comprising an array of image sensor pixels andreadout circuitry electrically connected thereto; and a secondsemiconductor chip coupled to the first semiconductor chip in a stackand comprising image processing circuitry electrically connected to thereadout circuitry; the readout circuitry comprising a plurality oftransistors each comprising spaced apart source and drain regions, asuperlattice channel extending between the source and drain regions, thesuperlattice channel comprising a plurality of stacked groups of layers,each group of layers comprising a plurality of stacked base siliconmonolayers defining a base silicon portion, and at least one oxygenmonolayer constrained within a crystal lattice of adjacent base siliconportions, and a gate comprising a gate insulating layer on thesuperlattice channel and a gate electrode on the gate insulating layer;wherein the superlattice channel is located entirely within the readoutcircuitry, and wherein no superlattice is located within the imageprocessing circuitry.
 19. The CMOS image sensor of claim 18 wherein thefirst semiconductor chip further comprises an electrical interconnectlayer beneath the array of image sensor pixels and defining a back sideillumination (BSI) configuration therewith, the electrical interconnectlayer electrically connecting the array of image sensor pixels with thereadout circuitry.
 20. The CMOS image sensor of claim 18 furthercomprising at least one lens overlying the array of image sensor pixels.21. The CMOS image sensor of claim 18 further comprising at least onecolor filter overlying the array of image sensor pixels.
 22. The CMOSimage sensor of claim 18 wherein the image processing circuitry alsocomprises a plurality of transistors each including a superlatticechannel.